The invention relates to a process for the ionization of samples of organic substances transported by a conveyor belt from a liquid chromatograph (LC) into the vacuum area of a mass spectrometer (MS).
The coupling or combination of mass spectrometers with other analyzing devices has been known. The combination of a mass spectrometer with a liquid chromatograph is particularly advantageous since as a result, the good separating effect of the liquid chromatograph coincides with the good specificity of substance of the mass spectrometer.
While in a known coupling of a gas chromatograph with a mass spectrometer a separation of the substances is carried out in the gaseous state, and therefore the good volatility of such substances is a prerequisite, the LC/MS coupling will also permit the analysis of difficult to vaporize substances. The prerequisite, however, is that these substances may be dissolved and ionized in the mass spectrometer. In particular, many bio-chemically and medically important compounds belong to groups of these difficult to vaporize substances.
The consecutive connection of an LC and MS has already been proposed. In the construction of such a combination one may, for example, proceed in such a way that the organic substance contained in a solvent is applied along with the solvent to a conveyor belt or wire which feeds the sample to a connected mass spectrometer. The conveyor belt leads up into the vacuum area of the mass spectrometer. This vacuum area is limited against the infeed area of the sample by underpressure sluices and suitable gaskets or packings. Prior to the entry of the sample into the vacuum area of the mass spectrometer the solvent is evaporated so that the dried sample on the belt is conveyed into the mass spectrometer.
The ionization of the organic sample reaching the mass spectrometer in this way constitutes a special problem. In this connection only electron impact ionization is known, which assumes that the sample evaporates in the vacuum area and is then ionized by electronic impact. The conveyor belt is properly heated for the evaporation of the sample.
Electronic impact ionization is particularly unfavorable or unsuitable in the investigation of sensitive organic substances. The molecule that is to be ionized absorbs internal energy twice in the case of this ionization process. Even during evaporation by heating, enough inner energy is absorbed so that a complete decomposition may occur in particularly sensitive substances. The additionally fed-in energy (beyond the ionization energy) during the electronic impact-ionization causes a further strong fragmentation of the molecules so that the mass spectra produced are no longer characteristic of the starting substance. For this reason, the LC/MS coupling combined with a conveyor belt has not yet gained acceptance in practice corresponding to the potential importance of such combination.